Such issues are usually a problem with hardware acceleration.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
• https://hacks.mozilla.org/2010/09/hardware-acceleration/

• https://support.mozilla.org/kb/how-do-i-upgrade-my-graphics-drivers

I have tried disabling hardware acceleration and that did not help. My graphics card drivers are up to date too.

Maybe an extension is causing that issue.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I have tried all of the basic troubleshooting steps listed here:Troubleshoot and diagnose Firefox problems. I have even tried it on a second computer with a fresh install of windows and Firefox. I still can't get it to stay full screen.

I can confirm this problem exists even in my Ubuntu Lucid 10.04 desktop machine. Using the latest firefox 12.+ thats available with lucid. When i go full-screen in one monitor and then click on the desktop or any other app in the other monitor then firefox automatically goes out of full-screen. The only "workaround" i found is to right click and then select "Pop out" on the video it opens within a maximized window and then I am able to click on the other desktop , but the video isn't exactly "full-screen" now. IMO it looks like a bug in the html5 full-screen code since it is being over-conservative and exiting even if you change focus to another monitor.

There is a fix for this, but apparently it isn't yet available in the current Firefox release (12.0). When the next update of Firefox comes out, you'll be able to go to about:config and set "full-screen-api.exit-on-deactivate" to false and that will fix the problem.

Hopefully Mozilla will soon make that the default so users won't have to do this!

https://bugzilla.mozilla.org/show_bug.cgi?id=724554

Cool, thanks for the info.

I have been having this issue forever, and have tried searching for an answer numerous times. Every answer seemed to blame Silverlight, Flash, etc. Finally, I found an answer that worked! I registered just to thank you!